This proposal is made up of three largely independent projects. (a) A continuation of a study, started in England, of the inducible DNA repair pathways found in E. coli (and in mammalian cells) that deal with the lesions produced by different classes of mutagen. (b) A continuation of a study of the patterns of DNA segregation observed in the stem cells of various vertebrate tissues. (c) A preliminary attempt, using appropriately marked prokaryotes, to determine what agents (if any) increase the rate of movement of transposons and therefore might be carcinogens, acting by producing regulatory transpositions.